Sommerfeld-enhanced annihilation in dark matter substructure: Consequences for constraints on cosmic-ray excesses

Abstract

In models of dark matter with Sommerfeld-enhanced annihilation, where the annihilation rate scales as the inverse velocity, N -body simulations of dark matter structure formation suggest that the local annihilation signal may be dominated by small, dense, cold subhalos. This contrasts with the usual assumption of a signal originating from the smooth dark matter halo, with much higher velocity dispersion. Accounting for local substructure modifies the parameter space for which Sommerfeld-enhanced annihilating DM can explain the PAMELA and Fermi excesses. Limits from the inner galaxy and the CMB are weakened, without introducing new tension with substructuredependent limits, such as from dwarf galaxies or isotropic gamma-ray studies. With substructure, previously excluded parameter regions with mediators of mass ∼ 1-200 MeV are now easily allowed. For O(MeV) mediators, subhalos in a specific range of host halo masses may be evaporated, further suppressing diffuse signals without affecting substructure in the Milky Way.PACS numbers: 95.35.+d

Secluded dark matter coupled to a hidden CFT

“…This limit requires the coupling constant α 0 to be smaller than that compatible with the results of Ref. [82]. The small-N limit does appear to be marginally consistent with their results, though only as k/M * → O(1), for which the perturbative gravity description begins to break down.…”

“…Indeed, in the split-fermion case with M DM ∼ TeV and Λ IR ∼ GeV, our theory provides a concrete example of a model with "new irrelevant annihilation channels" according to the definitions of Ref. [82]. However, we find that, even in the presence of large amounts of local substructure, the large-N limit seems incompatible with the successful generation of the cosmic-lepton excess.…”

“…Should large amounts of local substructure be present, the local DM velocity can be lower than the average value for a smooth halo, thus generating a larger local boost factor. It has been argued that such substructure can allow models of annihilating DM to generate the lepton excess and remain compatible 26 with the most robust constraints [82]. Indeed, in the split-fermion case with M DM ∼ TeV and Λ IR ∼ GeV, our theory provides a concrete example of a model with "new irrelevant annihilation channels" according to the definitions of Ref.…”

Sommerfeld enhancement from multiple mediators

“…If the DM has a mass-splitting, the situation is more promising due to the increased enhancement shown in (27) [24]. The existence of local substructure in the DM halo can also improve the fit to the data [25]. Note, however, that recent gammaray observations of the galactic center by HESS disfavor large regions of the parameter space, subject to the precise nature of the DM density profile 7 [26].…”

“…However, this feature is precisely that required if there are sizable amounts of local substructure. Indeed, the model falls under the category of "new irrelevant channels" according to the definitions of [25]. On the other hand, if the DM is produced non-thermally by inflaton decays or some other such mechanism, as may be needed if the early universe is not reheated above the IR scale Λ IR , the CFT modes need not be in equilibrium in the early universe.…”